Phosphor-enhanced light sources are known per se and are used for substantially all kinds of light sources. Phosphor-enhanced light sources comprise a light emitter and a luminescent material. The luminescent material is arranged for converting at least part of the light emitted by the light emitter into light of a longer wavelength.
Well-known phosphor-enhanced light sources are, for example, mercury vapor discharge lamps in which the light is emitted from a discharge in which the presence of mercury vapor causes the discharge to emit ultraviolet radiation. At least a part of the ultraviolet radiation is absorbed by a luminescent material and converted into light of a longer wavelength which is subsequently emitted by the luminescent material. Such mercury vapor discharge lamp may, for example, comprise a discharge vessel in which the discharge is generated. The luminescent material is typically applied to the inner wall of the discharge vessel such that the ultraviolet radiation emitted by the discharge does not need to pass the discharge vessel but is inside the discharge vessel converted into, for example, visible light.
Alternatively, the phosphor-enhanced light source may comprise a solid-state light emitter as the light emitter. Such a solid-state light emitter may, for example, be a light emitting diode, or a laser diode, or an organic light emitting diode. The light emitted by a solid-state light emitter typically has a relatively narrow spectrum arranged around a center wavelength. The width of the spectrum may, for example, be defined by the Full Width Half Maximum (further also indicated as FWHM) of the emission peak which is a width of the emission peak measured at an intensity being half the maximum emission intensity of the light emitted by the solid-state light emitter. The FWHM of a typical emission spectrum of the solid-state light emitter is less than 30 nanometer, which is typically identified by the human eye as light of a single color.
To change the color of the light emitted by the solid-state light emitter, luminescent materials may be added to generate a phosphor-enhanced light source. The luminescent material may, for example, be applied as a layer on top of the (LED) die of the solid-state light emitter, or may, for example, be dispersed in a matrix which may be located at a distance of the solid-state light emitter, a so called “remote phosphor” arrangement. The luminescent material may also be part of a mixture of different luminescent materials, for example, each generating a different color such that the mixed light, for example, generates white light having a specific color temperature. Furthermore, luminescent materials may be added to solid-state light emitters to improve the color rendering characteristics of the solid-state light emitters, as the typical emission characteristic of the luminescent materials is a relatively broad spectrum of light.
The use of dyes in matrices is (also) known in the art. U.S. Pat. No. 6,537,679, for instance, describes a fluorescent retro reflective article comprising a polymer resin comprising poly(1,4-cyclohexanedimethanol-co-ethylene terephthalate)(PETG) and a fluorescent dye selected from the group consisting of perylene imide and perylene ester dyes, thioxanthene dyes, benzoxanthene dyes, and benzothiazine dyes. The PETG fluorescent resin matrix can be used to enhance daytime visibility of a roadway marker. Such a pavement marker comprises a base member comprising a structure of a light-transmissible fluorescent material, the structure having a top surface and a front edge surface, the base member being configured to provide an air cap beneath the structure.
Qian Xuhong et al. (J. Chem. Eng. Data (1988, 33, 528-529) describes some benzoxanthene-3,4-dicarboximides and benzimidazoxanthenoisoquinolinones, and their physical and spectral data. JP06228549 describes an organic electroluminescent element with a compound as described by Qian Xuhong et al. EP2645822 describes a lighting device comprising a light source and luminescent materials. The luminescent materials comprising a first organic luminescent material, a second organic luminescent material, optionally one or more further organic luminescent materials, and optionally one or more further inorganic luminescent materials. The light source and the luminescent materials are configured to generate white lighting device light during operation. The first organic luminescent material degrades with time, the second organic luminescent material degrades with time, and the optional one or further organic luminescent materials degrade with time. The luminescent materials are configured to maintain the lighting device light white during operation time of the lighting device. Xuhong Qian et al. (Dyes and Pigments, vol. 32, no. 4, p. 229-235 (1996)) provides a study on the relationship between Stokes shift and low frequency halve-value component of fluorescent compounds, amongst others one described by Qian Xuhong et al.